Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid catalyst. Old ethylbenzene production plants, typically built before 1980, used AlCl3 or BF3 as the acidic catalyst. Newer plants have in general been switching to zeolite-based acidic catalysts.
Traditionally, ethylbenzene has been produced in vapor-phase reactor systems, in which the ethylation reaction of benzene with ethylene is carried out at a temperature of about 380-420° C. and a pressure of 9-15 kg/cm2-g in multiple fixed beds of zeolite catalyst. Ethylene exothermally reacts with benzene to form ethylbenzene, although undesirable chain and side reactions also occur. About 15% of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products. All these chain reaction products are commonly referred as polyethylated benzenes (PEBs). In addition to the ethylation reactions, the formation of xylene isomers as trace products occurs by side reactions. This xylene formation in vapor phase processes may yield an ethylbenzene product with about 0.05-0.20 wt. % of xylenes. The xylenes show up as an impurity in the subsequent styrene product, and are generally considered undesirable.
In order to minimize the formation of PEBs, a stoichiometric excess of benzene, about 400-2000% per pass, is applied, depending on process optimization. The effluent from the ethylation reactor contains about 70-85 wt. % of unreacted benzene, about 12-20 wt. % of ethylbenzene product and about 3-4 wt. % of PEBs. To avoid a yield loss, the PEBs are converted back to ethylbenzene by transalkylation with additional benzene, normally in a separate transalkylation reactor.
By way of example, vapor phase ethylation of benzene over the crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. No. 3,751,504 (Keown et al.), U.S. Pat. No. 3,751,506 (Burress), and U.S. Pat. No. 3,755,483 (Burress).
In recent years the trend in industry has been to shift away from vapor phase reactors to liquid phase reactors. Liquid phase reactors operate at a temperature of about 180-270° C., which is under the critical temperature of benzene (about 290° C.). One advantage of the liquid phase reactor is the very low formation of xylenes and other undesirable byproducts. The rate of the ethylation reaction is normally lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually economically compensates for the negatives associated with the higher catalyst volume. Thus, due to the kinetics of the lower ethylation temperatures, resulting from the liquid phase catalyst, the rate of the chain reactions forming PEBs is considerably lower; namely, about 5-8% of the ethylbenzene is converted to PEBs in liquid phase reactions versus the 15-20% converted in vapor phase reactions. Hence the stoichiometric excess of benzene in liquid phase systems is typically 150-400%, compared with 400-2000% in vapor phase.
Liquid phase ethylation of benzene using zeolite beta as the catalyst is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, see, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); and U.S. Pat. No. 6,231,751 (ITQ-2).
Although liquid phase ethylbenzene plants offer significant advantages over vapor phase processes, because they necessarily operate at lower temperatures, liquid phase processes tend to be more sensitive to catalyst poisons than their vapor phase counterparts, making them of limited utility with lower grade ethylene and benzene streams without significant feed pretreatment. However, the purification of alkylation feed streams is a costly business and hence there is considerable interest in developing processes that may operate with lower grade feed streams.
The present invention provides an aromatics alkylation process that allows the use of a dilute alkene feed, in which the aromatics feedstock is initially subjected to a vapor phase alkylation stage and then at least part of the unreacted aromatics feedstock is subjected to a liquid phase alkylation stage. In this way, the advantages of vapor phase alkylation, particularly decreased susceptibility to catalyst poisons, can be combined with the advantages of liquid phase alkylation, decreased capital cost and lower level of by-products. At least part of the effluent from the vapor phase alkylation stage undergoes interstage treatment to remove catalyst poisons before passing to the liquid phase alkylation stage.
U.S. Pat. No. 6,376,729 discloses a process for the production of ethylbenzene by the gas phase alkylation of benzene over a molecular sieve aromatic alkylation catalyst followed by liquid phase alkylation of the product of the gas phase alkylation. A feedstock containing benzene and ethylene is supplied to a first alkylation reaction zone containing a molecular sieve aromatic alkylation catalyst. The reaction zone is operated at temperature and pressure conditions to cause gas phase ethylation of the benzene with the production of an alkylation product comprising a mixture of ethylbenzene and a polyalkylated aromatic component including diethylbenzene. At least part of the output from the first alkylation reaction zone is supplied, without pretreatment, to a second alkylation zone which is operated in the liquid phase or in the supercritical region followed by supply to an intermediate recovery zone for the separation and recovery of ethylbenzene and a polyalkylated aromatic compound component including diethylbenzene.
European Patent No. 1,188,734 B1 discloses a process for the production of ethylbenzene similar to that disclosed in U.S. Pat. No. 6,376,729, except at least part of the polyalkylated aromatic component from the first gas phase alkylation reaction zone is reacted with additional benzene in a transalkylation zone and the effluent from the transalkylation zone is supplied to the second liquid phase alkylation zone.